Titanium and Ti6Al4V alloy are widely used as materials for orthopedic implants because of their advantageous mechanical properties and nontoxic behavior, Benjamin, D., editor. Metals handbook, Volume 3, 9th ed., Metals Park, Ohio: American Society for Metals; 1980, p 372-406; and Dobbs, H. S., Fracture of titanium orthopedic implants., J. Mater. Sci., 1982; 17: 2398-2340. However, one of the main drawbacks of using metallic implants is that they are bioinert and become encapsulated by dense fibrous tissue inside the body. This impedes proper stress distribution at the implant-bone interface, which can result in an interfacial failure and loosening of the implant with the possible consequence of fracture in the adjacent bone, Hench, L. L., Bioceramics: from concept to clinic, J. Am. Ceram. Soc., 1991; 74:1487-1510; and Suchanek, W. et al., Processing and properties of hydroxyapatite-based biomaterials for use as hard tissue replacement implants, Mater. Res., 1998, 13; 94-117.
In addition, the use of polymethylmethacrylate bone cement to improve implant fixation can result in the deterioration of the adjacent bone. There have been approximately 40 deaths in Japan allegedly associated to bone cements, “Bone Cement Kills 37 since 1987”, Pharmaceuticals and Medical Devices Safety Information No. 165, Mar. 30 (2001), Ministry of Health Labour and Welfare, Japan. A way to solve this problem and to improve the performance of metallic implants is to coat them with a bioactive material. A bioactive coating with good adhesion to the metal can also bond interfacially to the bone, which will accelerate the stabilization of the implant and extend its duration. Synthetic hydroxyapatite (HA) is very similar to the inorganic component of bone and it has proven to be bioactive, LeGeros, R. Z. et al., Dense hydroxyapatite In: Hench, L. L. et al., Editors: An introduction to bioceramics, Singapore: World Scientific; 1993 pp. 139-180; and Shors, E. C. et al., Porous hydroxyapatite: In: Hench, L. L. et al., Editors: An introduction to bioceramics, Singapore: World Scientific; 1993 pp. 181-198. Consequently, during recent years several groups investigated the fabrication of HA coatings on metallic implants; plasma-spray coating was the preferred technique, Lacefield, W. R., Hydroxylapatite coating In: Hench, L. L. et al., Editors: An introduction to bioceramics, Singapore: World Scientific; 1993 pp. 223-238; and Ha, S. W. et al., Chemical and morphological changes of vacuum-plasma-sprayed hydroxyapatite coatings during immersion in simulated physiological solutions, J. Am. Ceram. Soc. 1998; 81:81-88.
Reports about the performance of plasma-sprayed coatings seem to indicate that there is a faster adaptation of the bone to the implant and an appreciable improvement of the interfacial strength at the early stages. However, there is still some lack of data about the long-term efficiency of the plasma-sprayed coatings. Usually, the plasma-sprayed coatings consist of a mixture of amorphous and crystalline phases. The fast dissolution of the amorphous phase and some of the crystalline products like tricalcium phosphate degrade the stability of the coating. Further heat treatment to improve the crystallinity often results in cracking and loss of adhesion. Finally, plasma spray is a “line of sight” technique, which is not entirely suitable for coating implants that have complex shapes.
An alternative method is to coat the implant with a bioactive glass (able to form HA in vivo) that could provide the desired interfacial attachment to the bone. Several groups have attempted to coat metallic implants with bioactive glasses using enameling, rapid immersion in molten glass, or plasma-spraying techniques. Although some coatings with excellent in-vitro behavior were obtained, most of the glass coatings were marred by cracking and poor reliability at the glass-metal interface, Hench, L. L. et al., Bioactive glass coatings, In: Hench, L. L. et al., Editors, An introduction to bioceramics, Singapore: World Scientific; 1993 pp. 239-260; and Lee, T. M. et al., Characteristics of plasma-sprayed bioactive glass coatings on Ti6Al4V alloy: an in vitro study, Surface Coatings Technol. 1996, 79:170-177; and Kitsugi, T. et al., Bone-bonding behavior of plasma-sprayed coatings of Bioglass™, AW-glass ceramic, and tricalcium phosphate on titanium alloy. J. Biomed. Mater. Res. 1996, 30: pp. 261-260; and Chern, L. et al., Corrosion behavior of hydroxyapatite/bioactive glass plasma sprayed on Ti6Al4V, Mater. Chem. Phys. 1995 41: pp. 282-289. In particular, when coating Ti and Ti alloys, space problems result primarily from the generation of high thermal stresses, which arise from the difference in thermal expansion between the glass and the metal and the high reactivity of Ti with SiO2-based glasses. The reactions lead to the formation of brittle interfacial layers and gas bubbles in the coating.
Previous studies showed that it is possible to obtain bioactive glasses in the SiO2—CaO—MgO—Na2O—K2O—P2O5 system and glasses able to form HA in vitro with SiO2, contents as high as 57 wt %. These glasses present adequate softening points and thermal expansions close to Ti6Al4V (αTi6Al4V≈9.1-9.8×1O-6° C.−1 at 400° C.), Pazo, A. et al., HA-bioactive glass composites: high temperature reactivity and “in vitro” behavior, Scripta. Mater. 1996 34:pp. 1729-1733; and Pazo, A. et al., Silicate glass coatings on Ti-based implants, Acta. Mater. 1998: 46:pp. 2551-2558. Thus, they are candidates for use as coatings per se or mixed with other crystalline phases like HA as homogeneous or layered composites.